Single cavity slot dies are often used to coat compositions onto substrates because of their simplicity and ease of use. Center-fed single cavity slot dies are particularly simple in design and operation. However, when the coating composition to be applied contains suspended particulate material, such as nanoparticles, conventional powders, or microspheres or beads, difficulties can arise and coating defects can be introduced.
One such defect is center banding. When coating a substrate web, center banding manifests as a band of higher concentration of the particulate ingredient of the coating in a band down the center of the web, coincident with the center of the coating die. It is believed that center banding occurs because a center fed die has a higher velocity along the portion of the die that could be considered as a linear extension of the feed conduit. In other words, as the coating composition spreads out to fill the entire width of the die, its velocity in the machine direction is lower everywhere except for a channel down the centerline that coincides with the location of the center-fed die's feed inlet. Coating composition components that flow directly down this region along and near the centerline tend to not experience any sideward motion, and continue to move downstream at similar velocity to that they had while still in the feed inlet conduit, or at least at a velocity closer to the original feed inlet conduit velocity than that of composition portions farther to either side of the die.
This higher velocity near the centerline of the die is believed to cause an elevated concentration of particulates at the center of the coated substrate.
In some instances, variations in particulate concentration, bare streaks, or other defects may lead to undesired variations in optical clarity, appearance, or other parameters across the resultant coated article. For instance, coated layers comprising polymeric matrices with particles incorporated therein are used as haze or diffusion layers in some optical film applications. Variations in particle concentration in such products may be visible as undesired and disruptive variations in brightness, color, etc. of a device such as a digital display made with such products.
Many attempts have been made to eliminate center banding when center-fed single cavity slot dies are used for coating compositions that contain suspended particulates. One approach has been to redesign the shape of the die's cavity. But this approach has seen limited effect, and has the disadvantage that a die cavity must be redesigned specifically for each coating composition and intended coating speed. Thus, this approach lacks versatility.
Another approach has been to design for low die inlet flow velocities, but this approach has an inherent disadvantage in that it limits the potential coating speed of the coater. Other approaches relating to the formulation of the coating composition itself are also of limited applicability.
There remains a need for a versatile, simple, and cost-effective solution to avoid center banding when coating particulate-laden coating compositions with a center-fed single cavity slot die.